Is Driving High as Dangerous as Driving Drunk?

Q: I know all about the dangers of driving while drunk. But is driving while high on marijuana as dangerous?

Any form of driving while intoxicated is obviously a bad idea, but smoking a joint or taking an edible before getting behind the wheel can pose distinctive risks. That, experts say, is because of the particular ways that marijuana affects the brain, and the fact that there’s no standard dose for a federally criminalized drug.

With alcohol, there are universally accepted quantities for what constitutes a single drink: 12 ounces of regular beer, five ounces of wine and 1.5 ounces of distilled spirits. There’s no agreed-upon equivalent for cannabis, said Marlene Lira, a senior research manager at the Clinical Addiction Research and Education Unit at Boston Medical Center. This makes it tricky for consumers to measure how much they’ve actually consumed, and to assess how much cannabis is too much before heading on the road.

During the past decade, car accidents involving cannabis have been rising, and recreational use of the drug continues to climb. A recent analysis of U.S. public safety data showed that from 2000 to 2018, the percentage of motor vehicle fatalities involving cannabis more than doubled from nine percent to about 22 percent. By contrast, the percentage of fatalities involving alcohol stayed roughly the same during this period.

While there’s a dearth of data directly comparing the hazards of driving while high versus driving while drunk, the research that does exist suggests that marijuana may be less likely than alcohol to lead to deadly car crashes. In one 2017 examination of more than 4,000 drivers from a police database in France, researchers found that drivers under the influence of alcohol were roughly 17.8 times more likely to be responsible for fatal car crashes than drivers who were sober, while drivers under the influence of marijuana were 1.65 times more likely to cause deadly accidents. A study of U.S. roadside safety reports from 2007 estimated that drivers who used marijuana were 1.83 times more likely than sober drivers to be involved in a fatal crash.

Studies like these often rely on drivers who were drug tested after car crashes. But because cannabis can hang around in fatty tissues for up to 28 days, a positive drug test does not necessarily mean that a driver was high at the moment they were driving. This can lead to imprecise measures of how cannabis actually affects a person’s ability to drive, said Dr. Godfrey Pearlson, a neuroscience professor at Yale University who has researched marijuana and car safety.

It’s also tricky for marijuana users to predict how exactly they’ll be impaired, and for how long. If you smoke a joint or take a bong hit, you’ll feel high within minutes and then return to a base level after three or four hours, Dr. Pearlson said. If you opt for a pot brownie or a gummy, it takes longer for the high to kick in, since the marijuana in edibles has to be absorbed into your gut and be metabolized through your liver. And it’s difficult to predict just how long that high will last, Dr. Pearlson said.

“If someone takes an edible, the classic scenario is, they say, ‘Hey, I’m not feeling this,’ and then 40 minutes goes by, and they get in a car and start driving,” said Dr. Collin Reiff, a psychiatrist at NYU Langone Health, in New York City.

THC, the intoxicating substance in cannabis, impairs a number of behaviors related to driving, including the ability to weave between lanes. It also delays response times in general, Ms. Lira said.

The psychoactive effects of cannabis can also make driving dangerous. The paranoia that some people experience while high could induce a panic attack while on the road, Dr. Reiff said. Weed also can create an altered sense of time, the sludgy sense that everything around you is moving slower than it actually is. “There’s that joke: Why did you get pulled over?” he said. “If you’re speeding, it’s because of alcohol. If you’re going too slow, it’s because of cannabis.”

Cannabis also muddles the perception of what’s safe and what’s not. In a randomized trial of 191 regular cannabis users (people who used the drug an average of 16 times a month), researchers instructed participants to smoke THC cigarettes or placebos until they got as high as they wanted during a 10-minute time span, taking a minimum of four puffs. Thirty minutes later, and throughout the day, they were led into a series of driving simulations. Before each simulation, researchers asked the study participants to assess how high they were and how impaired they were to drive. After that first 30-minute interval between smoking and driving, just under half of the study participants who smoked THC said they felt safe to drive in their current state. But after 90 minutes, nearly 70 percent considered themselves OK to drive — even though the THC group performed just as badly (and worse than the placebo group) in the simulations.

“There’s this disconnect — after a while, they feel it wearing off,” said Thomas Marcotte, a professor of psychiatry at the University of California, San Diego, who led the study. “But in fact, you’re not necessarily a good judge at that point. You still should not go on the road.”